Technical notes of interest to Marine Engineers

Measuring water quality

Getting to know your instruments

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A while back, I was researching information about a micro Siemens meter that was used on a large fresh water generator I was responsible for. I came across this excellent article, although not marine influenced, give us an excellent overview of measuring devices often found in desalinization plant onboard ships.  - Martin

Electroconductivity, Parts Per Million (PPM), Total Dissolved Solids (TDS)

There are various methods of measuring the concentration of nutrient solutions. Although historically individual nutrients were measured, currently measurement devices measure a substance in solution that reflects total nutrient levels. Measurement devices used for this include Electroconductivity (EC) meters, CF meters, Parts per million (PPM, TDS) meters, &/or a combination of these. None of these methods provides an exact analysis of total nutrient levels; all are estimation, & are based on conductivity readings.

EC meters

Eectroconductivity (EC) express conductivity in metric units, & some read out in SI units. The Systeme Internationale (SI) is the international standard for the metric system. An example is the SI unit for length is the meter (m). Other metric units of length (centimeter, millimeter, etc.) are based on this SI unit.

Important electroconductivity terms are:

Resistance
Electrical resistance is the ability of a material to oppose the flow of an electric current. The SI unit of resistance, the Ohm, is designated by the Greek letter omega or "?".

Conductivity
Conductivity is the reciprocal of resistivity & measures the rate at which a small electrical current flows through a solution. The SI unit of conductivity of an element with a resistance of 1 ohm is Siemens (S). The unit was originally called Mho (Ohm spelled backwards), but has been replaced by S. The unit of conductivity is actually S/ centimeter (cm). Hydroponic electroconductivity (EC) is measured in milliSiemens/cm (mS/cm) or microSiemens/cm (S/cm). The prefix micro stands for 1/1,000,000. Importantly, the micro symbol is the Greek symbol Mu (); a u with a long leg in front of it. Important to note, the abbreviations for microSeimens & miliSiemens can be written in a similar manner. MicroSiemens per centimeter can be abbreviated m S/cm & milliSiemens per centimeter can be abbreviated mS/cm. The difference between the two one or no spaces between the m & S. These are different measurements since mili = 1/1000 & micro = 1/1,000,000. To avoid confusion, Mu () will be used.

Conductivity readings are based on the strength of electrolyte being measured. An electrolyte is a solute that dissociates (splits apart) fully or partially into ions when dissolved in a solvent, producing a solution that conducts electricity. A solvent is the most abundant substance in a solution. A solute is a substance which is dissolved in a solvent to make a solution. For example, when salt is dissolved in water, water is normally the solvent & salts are the solute. There are three strengths of electrolytes:

• A strong electrolyte is a solute that completely dissociates into ions in solution. & conducts electricity. Sodium chloride (NaCl) is a strong electrolyte & good conductor because it completely ionized to Na+ & Cl-. The greater the amount of Na+ & Cl- contained in water the more electricity is carried, & the greater the conductivity. Strong acids such as nitric acid (HNO3), & strong bases are also good conducting solutions.

• A weak electrolyte is a solute that incompletely dissociates into ions in solution. Since there is less ionization, most of the molecule stays together. The resulting solution contains both molecules & ions. Insoluble salts, & weak acids (i.e., vinegar) or weak bases form poorly conducting solutions. A solution of a weak electrolyte can conduct electricity, but usually not as well as a strong electrolyte because there are fewer ions to carry the charge from one electrode to the other.

• Non-electrolytes do not dissociate much at all. An example is sugar water.
An EC meter applies a weak electrical voltage to the solution across a pair of electrodes and reads the current that is produced from the electrolytes. EC is a measure of the total ion content of the nutrient. Since salt is a strong electrolyte, the higher the salt content, the greater the flow of electrical current. Increasing salt strength may not increase the conductivity, however. Salts reach a point where they become saturated & can dissolve no more. Although one can "supersaturate" the salts, methods of doing so (i.e., applying heat) are harmful to plants. At the saturation point ions act against each other, making it hard for electricity to flow.

Nutrient solutions contain various strengths of electrolytes and only ionized electrolytes are read by meters. This means that substances that are weak or non-electrolytes such as nitrogen or phosphorous containing compounds may not be read by the meters. Thus, EC does not give any indication of which nutrient ions are present, or information about undissolved nutrients; it estimates the overall available nutrient levels. Too high an EC results in vegetative growth with little fruit or flower production & too low an EC produces weak, unproductive plants.

CF meters

CF meters can be used to measure electroconductivity in CF units. Conductivity Factor (CF) is not a recognized scientific measurement, however. Meters are scaled from 0 to 100, where 0 represents pure water. The conversion of CF to Siemens is 1mS/cm = 10 CF. (Note: In reality, pure water conducts electricity & has a conductivity of < 1 S/cm. A very small part of the molecules of water are ionized as hydrogen ions & hydroxide ions).

Parts per million (PPM), Total Dissolved Solids (TDS) meters.

PPM refers to concentration expressed as parts of solute per million parts of solution. This normally refers to parts per million by mass. (Note: Mass = weight divided by 9.8 meters/second squared on planet earth). In very dilute solutions, ppm is approximately equal to milligram (mg) solute per liter of solution. PPM meters take an EC reading & use a conversion factor to convert the results to total dissolved solids (TDS). This reads out in parts per million (ppm). Importantly, different manufacturers may use different conversion factors, & different readings can be obtained from the same calibrating solution. To avoid this, a standard conversion factor of 0.64 is recommended.

Controversy arises as to whether an EC reading is more accurate than a ppm reading. Neither EC or PPM readings are exact; both are estimates. The only way to get an exact measurement of a nutrient is to measure that nutrient individually (i.e., ion chromatography) which is beyond the scope &/or necessity of most hydroponists. An understanding of PPM is necessary if one intends to mix their nutrient solutions.

EC, CF, PPM meter selection

• Desirable characteristics: Water proof, auto shut off, adjustable conversion factor.
• Accuracy: Accuracy, specified in terms of TDS, temperature, (EC) & resolution, depends on user need.
• Set point: Lets one set parameter so an alarm is activated if the solution goes outside of the parameter.
• Beta () is an adjustable temperature coefficient factor, not needed for hydroponics. Its use requires calculus skills.
• TDS & EC range: The range needed depends on the crop grown. One may not need the high range of an expensive meter if the crop they are growing has a low recommended EC or TDS. For example, the ppm of a bromeliad is very low (560-840 ppm); & if one were growing only bromeliads a meter with a capacity of 3000 ppm would be unnecessary.
• Temperature compensation feature: Nutrient temperature affects the ppm conversion factor. If the temperature deviates from 25 o C, the meter compensates with internal calculations. The EC value is converted to value as if the temperature were 25 oC. Meters may provide readouts in C & Fahrenheit (F) (C = (F-32)/ (5/9).
• Temperature range: Extreme meter temperature ranges may not be needed in temperate climates, but are necessary for extreme areas (i.e., desert areas).
• Electrode or probes may not be included with a meter & may need to be purchased separately. Probe warranties may be shorter than meter warranties; replacement probe cost should be investigated. Some hydroponic setups may need a longer electrode since placing an electrode in nutrient above its recommended depth can damage it.
• Warranty: Cheaper meters may have shorter warranties.
• Alternative/add on features: Computer software & devices such as Analog-to-Digital converters are used by some growers. These devices connect to pumps add nutrient as needed based on EC values. Computer supply sources can provide inexpensive devices.

Meter calibration

Standardized calibrating solutions, used for EC & PPM meters are:

• Sodium chloride (NaCl): 1000 ppm.
• Potassium chloride (KCL): 2760 S/cm or 1413 S/cm at 25o C.
• 442 mixture: Made from sodium sulfate (40%), sodium bicarbonate (40%) & sodium chloride (20%).

Calibration tips:

• Keep daily logs.
• Use a fresh solution & discard it once used.
• Avoid old nutrient; it may have concentrated salts.
• Calibration solution, electrode & nutrient temperature should be the same when taking measurements. A temperature compensation table is used to calibrate the meter at different temperatures.
• Use clean dry probes & follow manufacturer recommendations for cleaning. Clean after use to prevent a salt buildup.
• Store meters in a cool, dry place.
• Hard water can contribute to a high EC level. Tap water > 200 ppm is considered hard water. Water > 80 ppm CaCO3 is often treated with water softeners which adds salts to the nutrient. If hard water is suspected, contact the water department & obtain the annual water quality report. Useful information on the report includes ppm calcium carbonate (CaCO3, Mg, Ca, & Na & pH. Treatment methods include water filters, reverse-osmosis units, distillers, & de-ionizers. To test the efficiency of these methods, measure the conductivity before & after use.

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